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IC 8864 



Bureau of Mines Information Circular/1982 




The Relation of Geology 
to Mine Roof Conditions 
in the Pocahontas No. 3 Coalbed 

By Noel N. Moebs and John C. Ferm 




UNITED STATES DEPARTMENT OF THE INTERIOR 




4K 

Information Circular, 8864 



The Relation of Geology 
to Mine Roof Conditions 
in the Pocahontas No. 3 Coalbed 

By Noel N. Moebs and John C. Ferm 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



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This publication has been cataloged as follows: 



Moebs, Noel N 

The relation of geology to mine roof conditions in the Poca- 
hontas No. 3 coalbed. 

(Information circular / U.S. Department of the Interior, Bureau of 
Mines ; 8864) 

"Bureau of Mines coal mine health and safety program." 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:8864. 

1. Coal mines and mining— West Virginia— Pocahontas County- 
Safety measures. 2. Coal mines and mi ning— Virginia— Safety mea* 
sures. 3- Geology, Stratigraphic. 4. Ground control (Mining). 5. 
Rock bursts. I. Ferm, John C. II. Title. III. Series: Information 
circular (United States. Bureau of Mines) ; 8864- 

TN295.U4 622s [622'.334'0975487] 81-607819 AACR2 



CONTENTS 

Page 

Abstract. 1 

Introduction 1 

Roof rock characteristics and roof quality 3 

Rock types forming very poor roof 3 

Slump deposits 3 

Channel scours 4 

Fire clays 4 

Kettlebottoms 4 

Rock types forming poor-to-fair roof 5 

Crossbedded and pebbly sandstone 5 

Sandstone with shale streaks and interbedded shale and 

sandstone 5 

Crystallized sandstone and conglomerate 5 

Rock types forming good roof 6 

Massive gray sandstone 6 

Massive sandy shale 6 

Rock core photo manual 6 

Conclusions 8 

ILLUSTRATIONS 

1. Geographic location and generalized stratigraphic column of the 

Pocahontas No . 3 coalbed 2 

2 . Rock core photo manual 7 



THE RELATION OF GEOLOGY TO MINE ROOF CONDITIONS 
IN THE POCAHONTAS NO. 3 COALBED 

by 

Noel N. Moebs ] and John C. Ferm 2 



ABSTRACT 

Bureau of Mines studies of mine roof fall problems in the Pocahontas 
No. 3 Coalbed of southern West Virgania and southwestern Virginia have estab- 
lished that type and sequence of rock are significant factors in roof compe- 
tence. The poorest conditions occur where the immediate roof consists of 
slump structures and slickensided rock. The best conditions occur where the 
roof consists of a sequence that coarsens upward from shale to massive sandy 
shale. A small manual of color photographs of rock types was devised to aid 
in identifying drill cores. Proper identifications should enhance the 
prediction of areas of potential roof problems in advance of mining. 

INTRODUCTION 

Virtually every mine in the Pocahontas No. 3 coalbed of southern West 
Virginia and southwestern Virginia (fig. 1) has experienced unanticipated roof 
falls of undetermined origin. The cost of these falls is high in terms of the 
accident rate, cleanup, and lost production. 

In the interest of improved safety and efficiency in coal mining, studies 
were made of the geologic factors contributing to roof falls. The results, 
reported here, are based on work conducted under Bureau of Mines contract 
H0230028, through the Department of Geology, University of South Carolina, 



Geologist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, Pa. 
2 Professor of Geology, University of South Carolina, Columbia, S.C. (now with 



University of Kentucky, Lexington, Ky.). 



Columbia, S.C.; details of 
the investigations are pre- 
sented in an Open File 
Report. 3 

The initial study began 
with a preliminary reconnais- 
sance of numerous mines in 
the district, using a ques- 
tionnaire format for sys- 
tematic data collection. 
Statistical analysis of 
these data led to the selec- 
tion of five large operating 
mines as sites for a more 
detailed underground inves- 
tigation to determine the 
relation between roof qual- 
ity and rock type. This 
investigation consisted of 
geologic mapping of good and 
bad roof areas in each of 
the five mines and provided 
an opportunity to observe 
great variation in rock 
types and roof conditions. 
The results clearly pointed 
to some obvious relation- 
ships between rocks and roof 
quality. However, other 
factors — the effects of mining methods, mine layout, the proximity of surface 
effects such as stream valleys, and the presence of major faults on the margin 
of the coalfield — could not be disregarded entirely. 

As a consequence, a second study was instituted in a series of mines that 
were in close geographic proximity and in which mining methods were much alike. 
These similarities tended to negate the effects of depth of cover, major fault- 
ing, and other background attributes. In this second study, all accessible 
areas of the mine were examined with a standardized format for classifying 



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FIGURE 1. - Geographic location and generalized stratigraphic 
column of the Pocahontas No. 3 coalbed. 



3 Ferm, J. C, R. A. Melton, G. C. Cummins, D. Mathew, L. L. McKenna, C. Muir, 
and G. F. Norris. A Study of Roof Falls in Underground Mines on the Poca- 
hontas No. 3 Seam, Southern West Virginia and Southwestern Virginia. 
BuMines Open File Rept. 36-80, 1978, 92 pp.; available for consultation at 
Bureau of Mines facilities in Tuscaloosa, Ala., Denver, Colo., Avondale, 
Md., Twin Cities, Minn., Rolla, Mo., Reno, Nev. , Albany, Oreg., Pittsburgh, 
Pa., Salt Lake City, Utah, and Spokane, Wash.; U.S. Department of Energy 
facilities in Carbondale, 111., and Morgantown, W. Va.; National Mine 
Health and Safety Academy, Beckley, W. Va.; and National Library of Natural 
Resources, U.S. Department of the Interior, Washington, D.C.; and from 
National Technical Information Service, Springfield, Va., PB 80-158983. 



both rock type and roof quality. These data did not represent the wide spec- 
trum of rock types and roof conditions encountered in the first study, but 
they did provide a carefully controlled data set suitable for examining the 
roof competence characteristics of most common rock types. In both studies, 
underground mapping of roof conditions and rock properties were preceded by 
preparation of roof maps based on borehole data to determine the degree of 
precision with which roof rock types could be established in advance mining. 

From the study of drill core logs in conjunction with underground mapping, 
it was concluded that the descriptions of rock cores provided by drilling crews 
were so inadequate that their potential value in prediction of roof conditions 
was, for the most part, lost. To improve identification of rock cores, a small 
field manual was prepared. 4 Copies of this prototype core manual have been 
distributed to selected engineers and geologists for trial use and have proven 
to be very useful in drill core logging. 

ROOF ROCK CHARACTERISTICS AND ROOF QUALITY 

In using the results of the two underground studies, factors such as min- 
ing methods, entry width, pillar arrangement, and bolting pattern are assumed 
to remain essentially constant. Even the best roof will fail with adverse 
background conditions or poor mining methods. Where roof rock types blend 
laterally and vertically into one another to occur in a mixed sequence, it 
should be remembered that two very different rock types located close together 
form a natural, physical discontinuity in materials and hence a zone of 
potential weakness. 

There have been no controlled experiments to verify any of these results; 
as a consequence, they should be regarded primarily as a base for further 
experimentation or a general guide for operating personnel or engineers. 

Rock Types Forming Very Poor Roof 

Slump Deposits 

The worst roof in the entire study occurred in slump structures where 
large sets of closely spaced slickensided planes extended upward for at least 
30 feet into the roof, separating inclined blocks of shale, sandy shale, sand- 
stone, fire clay, and rider coal. Slump deposits can be detected in core 
drilling by the presence of steeply inclined bedding separated by slickensided 
planes, but with 1,000- or 2,000-foot drill hole spacing, small areas (200 or 

4 Ferm, j. C. , and R. A. Melton. A Guide to Cored Rocks in the Pocahontas 
Basin. University of South Carolina, Columbia, S.C., 1977, 89 pp. (This 
manual is out of print. A revised and improved version entitled "A Guide 
to Cored Rocks in the Southern Appalachian Coal Fields" is available from 
the Department of Geology, University of Kentucky, Lexington, Ky. A manual 
entitled "A Guide to Cored Rocks in the Pittsburgh Basin," similar in for- 
mat, was prepared under Bureau of Mines contract J018115 and is also 
available from the Department of Geology, University of Kentucky.) 



300 feet wide) may go undetected. It is expected that slumped roof rock would 
be a major problem in either room-and-pillar or longwall mining. 

Channel Scours 

Very poor roof conditions are commonly encountered near channel scours, 
where fine-grained rocks abut slickensided contacts on the flanks of trough- 
shaped, sandstone-filled channels (roof rolls). Because such contact zones 
are relatively narrow (in most cases, less than 100 feet), they are not often 
directly encountered in drilling. However, where substantial areas show 
sandstone roof and the remainder is shale or some similar fine-grained rock, 
such a contact zone can be expected, and its general location noted. In some 
cases, such contacts are also characterized by slumping, and intensive advance 
drilling may be necessary to delineate the slump deposits. 

Unless channel scours are parallel to entries, crosscuts, or longwall 
faces, they should present only local problems in room-and-pillar mining; 
however, those that extend into the coal seam may cause serious problems in 
longwall mining. 

Fire Clays 

Fire clays in the roof of the Pocahontas No. 3 Seam are uncommon. The 
degree to which they present roof problems is closely related to the grain 
size of the material and its position in the roof rock sequence. If the imme- 
diate roof consists of fine-grained fire clay, and particularly if it is cross- 
cut by slickensides, serious roof control problems can be expected. If, on 
the other hand, there are 2 to 4 feet of sandy fire clay directly above the 
coal, roof conditions will be considerably better, but widely separated slick- 
ensided planes may result in large "horsebacks," which, once disturbed, are 
difficult to support. 

Areas of fine-grained and sandy fire clays should be detectable with 
drilling on 1,000- or 1,500-foot spacing. However, careful mapping to show 
their exact position relative to the top of the coal should be done as mining 
into such areas progresses. Careful monitoring should indicate in advance the 
presence of clay directly over the coal and allow for planning additional 
support. 

Fire clays are considered very poor top for both longwall and room-and- 
pillar mining, but sandy fire clay may have some advantages in longwall mining 
as it should fall readily behind the supports. 

Kettlebottoms 

A kettlebottora is a cylindrical or inverted funnel-shaped rock mass 
imbedded in the coal mine roof rock. It consists of an erect or sloping mold 
of an ancient tree stump or root system. The margins of a kettlebottom are 
nearly always slickensided or marked by a thin layer of coaly material. 



Kettlebottoms are included in the very-poor-roof category although they 
are very small, local features of erratic occurrence, which cannot be detected 
by core drilling. When they are encountered during mining, the risk from 
falls can be reduced by inserting a diagonal bolt through the kettlebottom 
into the enclosing rock. 

Rock Types Forming Poor-to-Fair Roof 

Crossbedded and Pebbly Sandstone 

Many sandstones that are strongly crossbedded or contain zones of shale, 
ironstone or coal pebbles, or coal streaks produce weak roof. Such zones gen- 
erally occur in the lower part of sandstone units but are very erratically 
distributed, and their occurrence is not readily predicted by advance core 
drilling or detailed underground mapping. Room-and-pillar mining is more 
adversely affected by these rocks than longwall mining. In longwall mining, 
features such as pronounced crossbedding, pebbles or streaks of shale, coal, 
and ironstone enhance breakup characteristics and facilitate caving. 

Sandstone With Shale Streaks and Interbedded 
Shale and Sandstone 

These rocks, which consist of hard, durable sandstone interlayed with 
relatively weak shale, produce poor roof (called stackrock), and rocks with 
close spacing of soft, thick shale layers seem to be the most troublesome. 
Where interbedded shale and sandstone rocks are present, they are relatively 
widespread and can be detected by advance core drilling. Such rocks were 
observed only in room-and-pillar mining in the area studied, but favorable 
behavior can be anticipated in longwall raining where prompt caving is desired. 

Crystallized Sandstone and Conglomerate 

Roof rocks of this type were observed in the Pocahontas No. 3 Seam only 
in northern Buchanan County, Va., but their presence can be expected both 
north and west of this location. They are generally widespread, and core 
drilling on 1,000-foot spacing combined with ongoing underground mapping 
should clearly delineate their occurrence. These rocks are extremely hard and 
brittle and tend to break abruptly. Roof bolting is costly and time consuming 
because drilling bolt holes is very slow and requires frequent changing of 
bits. Moreover, there is some question as to whether the bolt holes weaken 
the rock more than the bolts strengthen it. The most serious problem with 
crystallized sandstone and conglomerate occurs in longwall mining when the 
caving behind the supports simultaneously releases methane and generates 
sparks as broken rock fragments collide. In the Buchanan County area, this 
problem has created ignitions resulting in serious delays in production. 



Rock Types Forming Good Roof 

Massive Gray Sandstone 

This rock type is a very common roof material over the Pocahontas No. 3 
Coalbed, and where it does not include pronounced crossbeds, shale or coal 
streaks, or shale or ironstone pebbles, it produces an excellent roof material. 
It forms a good top in room-and-pillar mining, but in longwall mining there is 
some tendency for the rock not to fall behind the supports. The presence of 
massive sandstone can be mapped using 1,500-foot drill hole spacing, but dur- 
ing mining, the presence of areas of shale, coal, or ironstone pebbles and 
streaks should be monitored. 

Massive Sandy Shale 

This rock type — a dark-gray shale that is gritty to the touch but without 
abundant sand streaks — makes up the best roof observed in the study area. In 
many cases, it occurs in a 15- to 20-foot sequence that begins with a thin, 
soft, dark-gray shale lying directly over the coal and grades upward into 
sandy shale. Such sequences are apparently very widespread, and reasonably 
accurate maps of their distribution can be prepared using corehole data. 
Relatively few problems have been encountered in either room-and-pillar or 
longwall mining in the mines studied. 

Other roof rock types have been found in Pocahontas No. 3 mines, but 
their occurrence is too infrequent to permit general conclusions concerning 
them. 

ROCK CORE PHOTO MANUAL 

One of the first characteristics of core log data that became apparent in 
the course of the underground studies was that, in general, the written log 
descriptions indicated in only an approximate way the variety of rocks actu- 
ally encountered, and many details relevant to roof behavior were omitted. 
Discussions with drilling and engineering personnel and observations of log- 
ging procedures indicated that many core loggers were concious of differences 
between rocks but lacked sufficient vocabularly to describe their observa- 
tions. Adding technical terminology without a fundamental understanding of 
its meaning only seemed to add to the" confusion. 

To increase the precision of core logging procedures, a manual was pre- 
pared, consisting of color photographs of the common rock types, together with 
names and code numbers that could be used to describe them (fig. 2). For the 
more abundant types, several examples were shown, and a simple key was pro- 
vided to assist in identification. In addition, data sheets were prepared so 
that notetaking could consist simply of recording a numeric code for rock 
types and any comments indicating special features qr modifications of terms 
and thicknesses of the rock types. Computer programs accepted these data for 
storage and retrieval and permitted conventional drillers' reports to be 
retrieved in printout form or graphic portrayal of the log. Details of the 




FIGURE 2. - Rock core photo manual. 



computer system are presented by Ferra. 5 This method of notetaking and record- 
ing data is now in use by several major coal companies and has made easier the 
task of determining the lateral distribution of rock types believed to be 
associated with bad roof conditions. 

CONCLUSIONS 

Studies of mines in the Pocahontas No. 3 Coalbed showed a relationship 
between geologic structure and mine roof quality. Slump deposits, channel 
scours, fine-grained fire clays, and kettlebottoms were found to be constitu- 
ents of very poor roof. Crossbedded and pebbly sandstone, interbedded shale 
and sandstone, and crystallized sandstone and conglomerate are poor-to-fair 
roof materials. Massive gray sandstone produces a good roof, and a rock 
sequence grading upward from shale to massive sandy shale makes up the best 
roof observed in these studies. 

Potential roof problems may be predicted in advance of mining, through 
drilling and identification of the rock cores. Although small or erratic fea- 
tures may be missed, most rock types are widespread enough to be detected by 
core drilling on 1,000-foot drill hole spacing. 

To make identification of the rock cores more precise and uniform, a 
small manual of color photographs of rock types and a simplified system of 
notetaking were prepared. The information obtained through this system can 
be used to designate potentially hazardous areas that will require extreme 
caution, supplementary roof support, deferred mining, or special extraction 
methods. The preliminary maps can be supplemented by monitoring of roof 
conditions as mining progresses. 

Such a system of preliminary mapping followed by monitoring should result 
in the improved application of geologic methods to coal mine ground control 
and greatly increase safety and efficiency in underground mining. 



5 Ferm, J. C. , and J. T. Berger. A Computer Graphics System Preparing Coal 
Bore Hole Data for Mapping. Computer Graphics, v. 2, No. 8, September 
1979, pp. 20-26. 
Hedge, S., J. T. Berger, and J. C. Ferm. An Interactive Computer System 
Preparing Borehole Data for Coal Seam Mapping. Trans. AIME, v. 268, 1980, 
11 pp. 

INT.-BU.OF MINES,PGH.,PA. 25806 



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